The present invention relates to a high-pressure discharge lamp having a light-transmitting air-tight discharge container, and an illumination device which uses the lamp.
High-pressure discharge lamps (to be called "ceramic discharge lamps" hereinafter) having discharge containers (to be called "light-transmitting ceramic discharge containers" hereinafter) made of light-transmitting ceramics are superior to conventional discharge containers made of quartz glass (to be called "quartz glass discharge containers" hereinafter) in terms of the heat resisting property and anti-corrosion property, and therefore they can achieve a high luminous efficiency and a high color rendition, as well as an excellent life duration property.
Further, light-transmitting ceramic discharge containers do not entail a phenomenon of the loss of clarity, which is caused by the reaction with light-emitting metals such as dysprosium Dy and sodium Na, and therefore they are capable of suppressing depression of luminous flux, which occurs due to the above phenomenon. Therefore, the ceramic discharge lamps are superior to high-pressure discharge lamp (to be called "quartz glass discharge lamp" hereinafter) equipped with a quartz glass discharge container in terms of the luminous flux maintenance factor.
However, while the inventors of the present invention were researching and studying a ceramic discharge lamp in order to have a higher luminous flux maintenance factor, they focused on that the luminous flux maintenance factor varies greatly within 100 hours of lighting.
FIG. 11 is a graph illustrating the luminous efficiency property with respect to the lighting time of the ceramic discharge lamp in four cases including commercially available ones and test samples.
In the figure, the abscissa axis indicates the time (hr) and the ordinate axis indicates the luminous efficiency (1 m/W).
In the figure, a curve A indicates the lighting time--luminous efficiency property of the first commercially available lamp, a curve B indicates that of the second commercially available lamp, a curve C indicates that of the first test sample, and a curve D indicates that of the second test sample. All of the ceramic discharge lamps are of a 150 W.multidot.3000K type, and the light-transmitting ceramic discharge containers, electrodes, sealing structures and discharge media of these lamps are designed under substantially similar conditions.
As is clear from the figure, in all of the ceramic discharge lamps, the reduction of luminous flux is prominent within 100 hours of lighting. Further, the lowering of the luminous flux maintenance factor in this period of time becomes even several tens of %. In extreme cases, within several minutes to several hours of lighting during the aging after completion of the manufacture, the ceramic discharge container blackens, and the luminous flux maintenance factor drastically decreases.
FIG. 12 is a graph illustrating the relationship between the entire luminous efficiency and luminous flux maintenance factor of an alumina valve which is a ceramic discharge container.
In this figure, the abscissa axis indicates the overall luminous efficiency (%) of the alumina valve and the ordinate axis indicates the luminous flux maintenance factor (%).
Further, in the figure, the overall luminous transmittance of the alumina valve of the ceramic discharge lamp and the change in the luminous flux maintaining factor until 100 hours of lighting are plotted.
As is clear from the figure, there is a clear correlation between the overall transmittance and the luminous flux maintenance factor, and the decrease in the luminous flux maintenance factor is caused by the blackening of the ceramic discharge container.
Under these circumstances, the inventors of the present invention analyzed the substance which causes the blackening, and discovered that the main component was carbon. In other words, as carbon precipitates on the inner surface of the ceramic discharge container, the blackening occurs.
Next, the source of carbon was investigated, and it was found that the sources were structural members such as electrodes, the ceramic discharge container and ceramics sealing compounds, and of these, carbon remaining on the electrodes was the main factor.
Further, a research was conducted to find out if the above-described blackening was a phenomenon unique to the ceramic discharge lamp, and it was found as a result that essentially the same phenomenon occurs in the quartz glass discharge container. However, even with the same electrode, and under the same conditions, the blackening is more prominent in the ceramic discharge container as compared to the quartz glass discharge container.
Furthermore, it was found as the results of the research and studies that the concentration of the impurities including carbon remaining on the surface of the electrode, and the like, is significantly related to the roughness of the surface of the electrode. More specifically, in the electrode of a high-pressure discharge lamp, containing tungsten as the main component, a wire material formed to have a predetermined width by the wire drawing method is used in general cases. During the drawing, a type of cut, which is called dies mark, is created, and a great amount of lubricant and polishing materials such as carbon and the like, remain in the mark of the cut.
Usually, a tungsten wire material obtained by the wire drawing is subjected to the high-temperature hydrogen process and the vacuum heat process, further, if necessary, a chemical polishing process. However, in practical cases, whether or not an irregularity on the surface and impurities created due to these processes are sufficiently eliminated from the surface, is not examined so intensely.
If carbon remains on the surface of the electrode to form WC or the like, the vapor pressure increases as compared to the case of pure tungsten, and the melting point decreases. Therefore, the amount of substance of the electrode scattered while lighting markedly increases.
In some cases, a mechanically polished wire which has been subjected to a so-called barrel polishing after forming an electrode by grinding is used; however alumina is used as the polisher, and alumina easily attaches to and remains on the surface of the tungsten wire material.
Alumina attached to the electrode reacts with quartz at high temperature in the quartz glass discharge container while lighting, to create alumina silicate, thus causing whitening in the discharge container. Further, alumina reacts with tungsten on the surface of the electrode while lighting, to form tungsten aluminate. Once tungsten aluminate is formed, the vapor pressure increases more as compared to the case of pure tungsten, and the melting point decreases. Therefore, the amount of the substance for the electrode, scattered while lighting, markedly increases. Further, if there are innumerable recesses and projections in the surface of the electrode after the completion of the above-described process, electron emission characteristic from the surface of the electrode and effective work function vary from a side to side on the surface of the electrode, and therefore it is considered that it causes the blinking of discharge.
The inventors of the present invention have found that if the concentration of impurities such as carbon and the like, which remain on the surface of the electrode, and the recesses and projections on the surface are controlled by setting the state of the surface of the electrode to predetermined conditions, the scattering of the substance for the electrode and the blinking of discharge can be significantly improved.
In the field of the high-pressure discharge lamp, the technique for improving the decrease in the luminous flux maintenance factor and the discharge blinking phenomenon, which are caused by the decrease in the light transmittance, which are due to the blackening, whitening or the loss of clarity, is disclosed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 5-86026.
However, the above-mentioned prior art technique, although an effect can be obtained to some extent, is not an essential countermeasure to the blackening caused by remaining carbon, but rather a secondary countermeasure (after treatment). Thus, the prior art technique is not an ultimate solution. As a result, the effect and stability of the degree which can be achieved by the prior art technique are not sufficiently satisfactory.